<div class="slider">

<ul>
<?php
$term = get_term_by('slug', get_query_var('term'), get_query_var('taxonomy'));
$sticky = get_option('sticky_posts');
    $args_cat_only = array (
        'my_cat' => $term->slug,
        'posts_per_page' => -1,
        'post__in' => $sticky,
        'post_type' => MYCPT_POST_TYPE,
        'post_status' => 'publish',
        'orderby' => 'rand',
    );
$cat_only_query = new WP_Query( $args_cat_only );

if ( $cat_only_query->have_posts() ) {
    while ( $cat_only_query->have_posts() ) {
        $cat_only_query->the_post(); ?>

        <li>
        // items from category only
        </li>

   <?php  } wp_reset_postdata();

if( count($cat_only_query->posts) <= 4 ) {

    $args_all_cats = array ( //Args to pull from all categories
        'orderby' => 'rand',
        'posts_per_page' => -1,
        //'my_cat' => $term->slug,
        'post__in' => $sticky,
        'post_type' => MYCPT_POST_TYPE,
        'post_status' => 'publish'
    );

    $cat_query_all = new WP_Query( $args_all_cats );

    if( $cat_query_all->have_posts() ) {
        while( $cat_query_all->have_posts() ) {
            $cat_query_all->the_post(); ?>

        <li>
        // items from ALL categories
        </li>

   <?php } wp_reset_postdata();
            }
        }
    }
?>
</ul>